The present invention relates generally to nuclear magnetic resonance (NMR) and, more particularly, to spatial localization in magnetic resonance imaging (MRI), chemical shift imaging (CSI), and magnetic resonance spectroscopy (MRS) using high-order magnetic field gradients.
It is highly desirable to be able to obtain an NMR spectra or an NMR image from a selected localized region within a sample body. Where the sample body is a living organism such as a human body and the like, various in vivo studies would be possible if a localized volume can be precisely selected from the ordinarily complex three-dimensional shape of the sample object. The important criteria sought to be achieved in NMR spatial localization are: (a) good selectivity, which includes uniformity and sharp edge definition of the selected region, and good suppression of NMR signals from outside of the selected region; (b) flexibility in choosing certain parameters such as echo time, and the location and shape of the selected volume; and (c) the use of fewer radio frequency (RF) pulses for multi-dimensional spatial localization, which minimizes selection artifacts, reduces the minimum echo time and decreases the total RF energy applied to the sample body.
Various techniques for spatial localization of NMR signals have been proposed. The most commonly used techniques, which involve the application of combinations of selective RF pulses and linear magnetic field gradients, provide reasonably good selectivity of a localized volume in the sample body. However, these techniques all require multiple RF pulse and magnetic field gradient sequences or iterations since localization in only one dimension can be selected by each combination of an RF pulse and a linear magnetic field gradient.
A typical example of spatial localization of NMR signals using the combination of a selective RF pulse and a linear magnetic field gradient is illustrated in FIG. 1. Referring to FIG. 1, the diagram 100 depicts the selection of a linear region 101 along the x axis by applying a linear magnetic field gradient G.sub.x along the x direction concurrently with the application of an RF pulse having a frequency spectrum 102 selected to excite spins only in the linear region between x.sub.1 and x.sub.2. The concurrent application of the magnetic field gradient 103 and the frequency-selective RF pulse having frequency spectrum 102 produces a magnetic resonance signal (e.g., spin echo or free induction decay) distribution in the x direction represented by the curve 104.
The diagram 110 depicts the selection of a two-dimensional region 111 in the x-y plane using combinations of RF excitation pulses and linear magnetic field gradients. To select the region 111, the RF excitation pulse having frequency spectrum 102 and the linear magnetic field gradient 103 are first concurrently applied to the sample object. Thereafter, another appropriate frequency-selective RF excitation pulse (not shown) is applied concurrently with a linear magnetic field gradient in the y direction (not shown) to define the boundaries y.sub.1 and y.sub.2. As may be seen from the example of FIG. 1, spatial localization using linear magnetic field gradients requires a separate RF excitation pulse for a selection in each dimension. Different variations of NMR spatial localization using combinations of RF excitation pulses and linear field gradients are described, for example, in U.S. Pat. No. 4,733,185 to Bottomley; and P. Mills et al., "Localized Imaging Using Stimulated Echos," Mag. Res. Med., Vol. 5, pp. 384-389, 1987; X. Hu, "SLIM: Spatial Localization by Imaging," Mag. Res. Med., Vol. 8, pp. 314-322, 1988.
Another expedient which is commonly used in NMR spatial localization is to apply the radio frequency fields to the sample body using a surface coil, i.e., a coil located at the surface of the sample body. However, the use of a surface coil to apply the RF field to the sample body tends to provide relatively coarse resolution of the selected volume, and the sensitivity of NMR signal detection decreases rapidly away from the sample body surface.
The use of high-order magnetic field gradients for NMR spatial localization is known. U.S. Pat. No. 4,651,098 to Yamada et al. proposes high-order gradients having the following functional forms for spatial localization in NMR imaging: (1) x.sup.2 +y.sup.2 +z.sup.2 ; (2) h.sub.1 (x.sup.2 +y.sup.2)+h.sub.2 z.sup.2, where h.sub.1 &gt;0, h.sub.2 &gt;0, h.sub.1 .noteq.h.sub.2 ; and (3) h.sub.1 (x.sup.2 +y.sup.2)+h.sub.2 z.sup.2, where h.sub.1 &gt;0 and h.sub.2 &lt;0, or h.sub.1 &lt;0 and h.sub.2 &gt;0. Of the three types of high-order gradients proposed by Yamada, only the one having functional form (3) can be formed in free space, without the addition of auxiliary field components, provided that h.sub.1 =-h.sub.2 /2. In the Yamada method, one RF excitation pulse is applied with either a high-order magnetic field gradient or a linear magnetic field gradient to select a "planar region". Thereafter, an imaging sequence of encoding and read gradients are applied followed by data sampling with no further selective RF excitation pulse being applied to the sample body. If high-order magnetic field gradients having functional forms (1) (2) could be formed in free space, a spherical or ellipsoidal volume, respectively, would be selected and could be used to generate magnetic resonance images by scanning spherical volumes or by using a linear gradient to resolve voxels along the ellipsoid. However, high-order magnetic field gradients having functional forms (1) or (2) cannot be formed in free space without the addition of auxiliary field components which would alter the shape of the selected volume from that of a sphere or an ellipsoid, respectively. Although a high-order magnetic field gradient having functional form (3) may be formed in free space, the application of an RF excitation pulse together with such a together with such a high-order magnetic field gradient selects a non-planar region of non-uniform thickness, which is not useful for volume-selective MRS, MRI or CSI purposes without a further complex selection procedure. The functional form (3) with h.sub.1 =-h.sub.2 /2 can be expressed as z.sup.2 -(x.sup.2 +y.sup.2)/2. A high-order magnetic field gradient of such functional form is uniform in the z=0 plane, but becomes increasingly non-uniform as z increases or decreases from the value 0.
More recently, another NMR spatial localization technique using a radial gradient is described in S. Y. Lee et al., "Localized Volume Selection Technique Using an Additional Radial Gradient Coil", Mag. Res. Med., Vol. 12, pp. 56-63, Oct. 1989. According to the Lee technique, a circular region is selected by applying a radial magnetic field gradient in the x-y plane in conjunction with an RF excitation pulse. The radial magnetic field gradient is generated using a one loop main coil located in the z=0 plane in combination with a pair of Helmholtz coils which are used to cancel non-zero fields at the center of the main coil. Since a single-loop coil located in the z=0 plane generates many auxiliary field components, including a z.sup.4 component, in addition to a second order gradient component, it is extremely difficult using the Lee technique to move, or modify the shape or orientation of the selected volume.
Accordingly, a need clearly exists for an NMR spatial localization technique which overcomes the foregoing deficiencies of the prior art in providing: (i) improved uniformity and edge definition of the selected volume, and improved suppression of magnetic resonance signals from outside of the selected volume; (ii) improved flexibility in choosing the selection parameters, including the location, direction and shape of the selected volume; (iii) multidimensional selection using fewer RF pulses to achieve volume selection in a shorter time with minimal selection artifacts and with lower total RF energy being applied to the sample body; and (iv) improved simplicity in generating "zoomed" images without having to perform high-resolution imaging on the entire sample body.